The present invention relates to magneto-optical recording media, such as a magneto-optical disk, a magneto-optical tape, and a magneto-optical card, for use in magneto-optical recording and reproducing devices.
One example of a rewritable optical recording medium which has been conventionally put to use is a magneto-optical disk which is realized by a magneto-optical recording medium. In such a magneto-optical recording medium, information is recorded or erased by projecting a converged light beam on the optical recording medium from a semiconductor laser, so as to raise the temperature of a portion of the magneto-optical recording medium irradiated by the laser beam. Reproduction of recorded information is carried out by projecting a converged light beam on the magneto-optical recording medium with such an intensity which is strong enough to reproduce information but not to record or erase information, and by recognizing a polarization state of the reflected light.
However, a problem of such a conventional magneto-optical recording medium is that reproducing characteristics suffer when the size or intervals of recorded bits of magnetic domains become smaller than a spot size of the light beam. This problem is posed when the beam spot of the light beam converged on a target recorded bit also falls on an adjacent recorded bit. As a result, information cannot be reproduced separately from the individual recording bits.
A magneto-optical recording medium which intends to solve the foregoing problem is disclosed in Japanese Unexamined Patent Publication No. 180276/1997 (Tokukaihei 9-180276) (published date: Jul. 11, 1997) (U.S. Pat. No. 5,777,953). The magneto-optical recording medium proposed in this publication includes: a read-out layer which has an in-plane magnetization at room temperature and shifts with rise in temperature to perpendicular magnetization at a critical temperature; a non-magnetic intermediate layer made of a transparent dielectric material; a perpendicular magnetization film; and a recording magnetic layer, which are stacked in this order.
Further, for the purpose of improving reproducing characteristics of this magneto-optical recording medium, Japanese Unexamined Patent Publication No. 320134/1997 (Tokukaihei 9-320134) (published date: Dec. 12, 1997) (U.S. Pat. No. 5,939,187) proposes a magneto-optical recording medium which includes: a read-out layer which has an in-plane magnetization at room temperature and shifts with rise in temperature to perpendicular magnetization at a critical temperature; an in-plane magnetization layer having a Curie temperature in the vicinity of the critical temperature; a non-magnetic layer; a perpendicular magnetization film; and a recording layer for recording information.
Further, Japanese Unexamined Patent Publication No. 180486/1996 (Tokukaihei 8-180486) (published date: Jul. 12, 1996) (U.S. Pat. No. 5,659,537) proposes a magneto-optical recording medium which includes: a read-out layer made of a perpendicular magnetization film; a non-magnetic intermediate layer; a perpendicular magnetization film; and a recording layer for recording information.
In the magneto-optical recording media disclosed in the foregoing publications Tokukaihei 9-180276 and Tokukaihei 9-320134, the read-out layer has an in-plane magnetization until it shifts with rise in temperature to perpendicular magnetization at a critical temperature.
When the read-out layer has an in-plane magnetization, the information of the magnetic domains recorded in the recording layer is not transferred to the read-out layer and the information of the recorded magnetic domains is not reproduced.
At the critical temperature or higher temperature, the read-out layer shifts to perpendicular magnetization and the information of magnetic domains recorded in the recording layer is transferred to the read-out layer to be reproduced. Therefore, as long as the reproducing power of the light beam and the critical temperature at which the read-out layer shifts to perpendicular magnetization are properly set, it is possible to reproduce information separately from individual recorded bits, even when the beam spot of the light beam converged on the read-out layer falls on an adjacent recorded bit. That is, information can be reproduced according to a magnetic super resolution reproducing scheme which enables reproduction of information recorded in high density.
Further, the magneto-optical recording medium disclosed in Tokukaihei 8-180486 realizes magnetic super resolution reproducing by the transfer of magnetized information from the recording layer to the read-out layer which occurs only in a temperature increased portion where a magneto-static coupling force between the two layers has increased by the increased total magnetization of these layers as a result of a temperature increase.
However, despite this advancement, a larger recording capacity is demanded for the optical disk. In order to meet this demand, the recording layer needs to be provided with smaller magnetic domains and transfer these magnetic domains to the read-out layer, so that the information can be stably reproduced.
The recording layers of the magneto-optical recording media disclosed in the foregoing publications Tokukaihei 9-180276, Tokukaihei 9-320134, and Tokukaihei 8-180486 are all magnetic layers which have a compensation temperature in the vicinity of room temperature. Further, the magnetic layers of these publications, in addition to recording and carrying information, serve to generate a leakage magnetic flux for transferring the information of magnetic domains recorded in the recording layer to the read-out layer when reproducing information.
The Curie temperature of the recording layer is set to around 250xc2x0 C. to save recording power. However, in order to reduce cross-light from an adjacent track, the compensation temperature of the recording layer is preferably set to around 50xc2x0 C. to 100xc2x0 C. or higher temperatures to maximize the coercive force in the vicinity of the Curie temperature.
However, the increased compensation temperature with a fixed Curie temperature reduces a total magnetization of the recording layer as the compensation temperature is increased. This results in smaller leakage magnetic flux from the recording layer and thus weaker magneto-static coupling force between the recording layer and the read-out layer. This causes the problem of instable transfer of a magnetized state. Therefore, in a super resolution magneto-optical recording medium in which the read-out layer and the recording layer are magnetostatically coupled, it is difficult with the use of the recording layer alone to realize desirable recording characteristics with reduced cross-light, and a stable magneto-statical coupling state at the same time.
An object of the present invention is to reduce cross-light from an adjacent track and to stably copy a magnetized state.
In order to achieve this object, a magneto-optical recording medium according to the present invention at least includes: a recording layer, made of a perpendicular magnetization film, in which magnetized information is recorded in the form of a perpendicular direction of magnetization; a magnetic flux generating layer, made of a perpendicular magnetization film, which is exchange-coupled with the recording layer; and a read-out layer, having a perpendicular magnetization at a predetermined temperature or higher temperatures, which copies the magnetized information of the recording layer by magneto-static coupling with the magnetic flux generating layer and the recording layer, wherein the magnetic flux generating layer is a magnetic film having a lower Curie temperature than that of the recording layer and having a larger total magnetization than that of the recording layer at a temperature at which the read-out layer has a perpendicular magnetization.
According to this arrangement, the function of generating a leakage magnetic flux for magneto-static coupling with the read-out layer and the function of recording information, which have been conventionally performed by the recording layer alone, can be rendered separately to the magnetic flux generating layer and the recording layer, respectively. This enables the recording layer to have a high compensation temperature (preferably 50xc2x0 C. to 100xc2x0 C., or higher) so as to increase the coercive force in the vicinity of the Curie temperature. As a result, the adverse effect of cross-light from an adjacent track can be alleviated. Further, according to the foregoing arrangement, a large magnetic flux can be generated from the magnetic flux generating layer having a larger total magnetization than that of the recording layer at temperatures at which the read-out layer has a perpendicular magnetization. As a result, the magneto-static coupling force with the read-out layer can be increased to realize stable transfer of magnetized information.
Here, it is required that the magnetic flux generating layer has a lower Curie temperature than the recording layer because recording of information would otherwise be determined by the magnetic flux generating layer with the higher Curie temperature. In this case, information cannot be recorded accurately because the magnetic flux generating layer has a relatively low compensation temperature for successful magneto-static coupling with the read-out layer and therefore gradually loses its coercive force toward the Curie temperature.
With the foregoing arrangement, however, the compensation temperature of the recording layer can be increased to increase the coercive force and thereby alleviate the adverse effect of cross-light from an adjacent track, while ensuring stable and accurate transfer of magnetized information (magnetized state).
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.